The present invention relates to a liquid crystal display device for use as a display unit, such as a viewing display or projection display.
Hitherto, there has been known an active matrix liquid crystal display using a thin film transistor wherein an additional capacity electrode is provided in parallel relation to a gate bus line in order to restrain the application of a DC field to liquid crystal molecules which may result from an optimum-opposite-voltage deflection attributable to a capacity change due to the movement of liquid crystal molecules. Liquid crystal displays of this type have a drawback that the aperture ratio is proportionally lowered by the area of the additional capacity electrode. In an attempt to eliminate this drawback, it has been proposed that an additional capacity electrode is formed of a transparent conductive film (Japanese Patent Application Laid-Open Publication No. HEI 5-289111) as shown in FIG. 2. The liquid crystal display device shown in FIG. 2 includes a gate electrode 25 formed on a glass substrate 21, a gate insulating film 26 formed over the gate electrode 25 and glass substrate 21, and a thin film transistor 40 containing the gate electrode 25, with a pixel electrode 31 formed over a portion other than the thin film transistor 40 region containing the gate electrode 25. Further, the liquid crystal display device includes a passivation film 32 formed on the pixel electrode 31, and an additional capacity electrode 33 comprised of the transparent conductive film which is formed on the passivation film 32. The pixel electrode 31, additional capacity electrode 33, and the passivation film 32 interposed between the two electrodes 31, 33 constitute an additional capacity section. The device has liquid crystal 30 filled between the glass substrate 21 and a glass substrate 22 on which is formed an opposite electrode 23. Unfortunately, however, the liquid crystal display device having the additional capacity electrode comprised of a transparent conductive film poses a problem such that the transparent conductive film formed at an early stage of the manufacturing process is liable to a property change and the like at a later stage which may be a cause of misalignment in the process of semiconductor manufacture.
Therefore, with a view to solving this problem there has been proposed a liquid crystal display device having an additional capacity electrode formed after formation of the gate electrode, the additional capacity electrode being comprised of a transparent conductive film. FIG. 3 shows, as one example of such a liquid crystal display device, a partial sectional view of an active matrix liquid crystal display device incorporating a low-temperature poly-silicon thin film transistor. In FIG. 3, reference numeral 1 designates a glass substrate; 2 designates a poly-silicon semiconductor layer formed on the glass substrate 1; 3 designates a phosphorus ion doped poly-silicon region of the poly-silicon semiconductor layer 2; 4 designates a gate insulating film formed on the glass substrate 1 as well as on the poly-silicon semiconductor layer 2; 5 designates a gate electrode formed on the gate insulating film 4 on the poly-silicon semiconductor layer 2; 6 designates an interlayer dielectric film formed on the gate insulator film 4 and on the gate electrode 5; 7 designates a source electrode; 8 designates a drain electrode; and 9 designates an organic interlayer dielectric film formed on the interlayer dielectric film 6, and on the source electrode 7 and drain electrode 8. An additional capacity electrode 13 is formed between the gate electrodes 5, 5 on the organic interlayer dielectric film 9, then an inorganic interlayer dielectric film 14 is formed on the organic interlayer dielectric film 9 and on the additional capacity electrode 13, and then a pixel electrode 15 is formed on the inorganic interlayer dielectric film 14 and in a contact hole 19 provided on the drain electrode 8. In this way, the additional capacity electrode 13 is formed from a transparent conductive film after formation of the gate electrode 5, whereby it is intended that any deficiency such as property change of the transparent conductive film is prevented so that good matching can be assured in the process of fabrication.
However, in such a liquid crystal display device wherein the additional capacity electrode 13 comprised of a transparent conductive film is formed after formation of the gate electrode 5, there is no element for insulating the pixel electrode 15 and the opposite electrode (not shown) from each other, and this poses the problem of leak occurrence and/or short circuiting between the pixel electrode 15 and the opposite electrode.